1. Field of the Invention
The invention is related to circuits, and in particular to systems and methods for non-intrusive testing communication signals between circuits.
2. Description of Related Art
Electronic devices often communicate with one another to cooperatively perform an intended function. Different functional components are sometimes implemented on distinct circuit boards that communicate with one another to provide the desired function. The distinct boards communicate over one or more defined signal paths. Often, an interconnect board connects multiple circuit boards to couple the signal paths, thereby facilitating communication between multiple circuits implemented on separate boards. For example, a bus back-plane may be used to interconnect defined signals between two or more circuit boards coupled to the back-plane interconnect board. Discrete control lines and discrete status lines implemented on the interconnect board interconnect the multiple circuits. Discrete control and status lines may serve as reset signals, watchdog, and in-place signals, among other purposes. In-place signals may allow processing elements on each board to determine whether other systems contain alternate processing elements. Reset signals may allow each circuit board to reset the alternate circuit board to a known state if other communication paths fail. Watchdog signals may be used between circuits to allow one circuit to monitor the operation of the other circuit.
FIG. 1 illustrates an exemplary interconnected system 100 as presently known in the art. Interconnected system 100 includes first circuit board 101, second circuit board 102, and interconnect board 107. Interconnect board 107 is sometimes referred to as a mid-plane or as a back-plane. First functional circuit 103 is implemented on first circuit board 101 to perform intended functionality. Second functional circuit 104 is implemented on second circuit board 102 to perform intended functionality. First functional circuit 103 is coupled to second functional circuit by first path 105 and second path 106. Circuit 103 may generate a signal to be applied to second circuit 104 via path 105. Second circuit 104 may generate a signal to be applied to first circuit 103 via path 106. For example, each circuit may generate a reset signal to be applied to the other circuit in accordance with desired functionality. Often such signals are in pairs such as shown in FIG. 1. Each signal path is used identically in one of the two circuits. Often such mirrored pairs of signal paths are referred to as “cross-coupled” signal paths or cross-coupled signals.
Circuits require testing of such interconnect signals to maintain and ensure proper operation of the interconnect signals paths. For example, where each circuit monitors operation of the other (such as by use of a watchdog periodic signal transmission) and may reset a failed circuit (such as by use of a reset signal), these signal paths should be periodically tested to confirm their proper operation.
For first circuit board 101 to test second circuit board 102 requires active circuit components implemented on interconnect board 107. However, many system designs discourage the use of active circuit components on interconnect boards to test other circuits because active circuit components make diagnosing mechanical circuit problems with discrete lines more difficult (i.e., testing to detect broken signal traces or bent connector pins). Further, active circuits on such interconnect boards may interfere with specified signal timing requirements by imposing additional delays or other types of signal interference or distortion. Other solutions have included active circuits on the communicating circuit boards that coordinate with the functional logic of the communicating circuit boards to effectuate testing of signals exchanged therebetween. Such integration of testing features with functional circuit designs adds complexity in that existing functional circuit designs (i.e., standard components or standard cells) may require modifications.
It is evident from the above discussion that a need exists for improved designs to permit non-intrusive testing of interconnected circuit signal paths (i.e., cross-coupled signal paths).